During tubular running or makeup/breakout operations, the top drive and a rig floor device must work in tandem to ensure safe casing running, that is, at least one of them must engage and grip the tubular and tubular string at any given time during casing assembly or disassembly. Typically, an operator located on the rig platform controls the top drive and the floor device by inserting or removing slips by hand, or with manually operated levers that control fluid power to slips that cause the top drive and floor device to retain the tubular. At any given time, an operator can inadvertently drop the tubular by executing the wrong operation (i.e., moving the wrong lever or incorrectly reading the controls, improper installation of a gripping device, etc.). Extensive damage and delays in a drilling operation can result. There have been some attempts at improved safety through the use of added interlock systems (See, e.g., U.S. Pat. Nos. 6,742,596; 7,073,598; and U.S. Publication No. 2008/0264648), but these are deficient for one or more reasons, such as inaccurate signals from a conventional interlock system or a latch that doesn't properly hold the weight of a tubular.
There is a need therefore, for an interlock system adapted for use with a top drive and floor device to prevent an accidental release of a tubular. There is thus a need for an interlock system that prevents a rig floor device or a top drive from disengaging a tubular or tubular string when these components are not gripped.